This film strains so badly to be a black comedy that it does itself an injury. Aside from the general lack of funny gags, miscasting is the major problem. There's also an unwillingness to commit to the characters' dark sides, opting instead for gross-out silliness.

After her rich fiance dumps her, slacker teacher Elizabeth (Diaz) returns to her middle school classroom with even less interest than before, fending off the interest of hang-dog coach Russell (Segel), perky colleague Amy (Punch) and gullible Lynn (Smith), while the oblivious principal (Higgins) fails to notice that she's merely showing school-themed movies to her students. But she finds some motivation in stealing money for a boob job to impress a rich new teacher (Timberlake) and conning a test official (Lennon) so she can win a test-score prize.

From the start, it's obvious where the plot's rom-com element is heading, so that tension is removed entirely. Most of the comedy instead comes from Elizabeth's bad-girl behaviour, which is sometimes very funny but never develops any sense of consistency, perhaps because we can't quite believe that Diaz is actually a bad girl. She undercuts the nastiness with sass, radiating intelligent sarcasm rather than cruelty or rebellion. Honestly, would someone this smart be so focussed on such a superficial quest?

Meanwhile, Timberlake is trying a little too desperately to be a sexy nerd, never quite convincing us that he's really this dorky. Segel is slightly better at playing his sexy rogue, although he's a bit too nice, really. Essentially, this means that Punch walks off with the film in what turns out to be the villain role: a seemingly nice woman with an insane evil streak. She's much more consistently hilarious, with all of the best lines and a terrific sense of physicality as well.

In other words, Diaz should have swapped roles with Punch, and Timberlake with Segel. But there are other problems with the film, including the wildly swerving tone from grisly gross-out jokes to absurd wackiness to painful slapstick, but all without the slightest hint of irony or cynicism. It's essentially a warm-hearted movie about selfish, manipulative people who are impossible to like or care about.